The discovery of the Secually Dimorphic Nucleus of the Preoptic area (SDN-POA), a marked, hormone dependent anatomical sex difference in the rat brain, provides the opportunity to study fundamental mechanisms underlying the morphological, and presumably functional, sexual differentiation of the brain. The technique of tritiated thymidine autoradiography will be used to define the temporal pattern of origin of the neurons of the SDN-POA. Moreover, since it is possible to label specifically the neurons of the SDN-POA by exposing the rat to tritiated thymidine on day 18 of gestation, it will be possible to define anatomically the origin of the neurons of the SDN-POA and their pathway of migration to this mucleus, as well as to quantify neuronal survival during the phase of programmed cell death. Possible sex differences and steroid hormonal effects on these developmental parameters will be assessed. With the technique of steroid autoradiography we will determine when during perinatal development the neurons (or neuroblasts) of the SDN-POA attain the capacity to take up and retain labelled steroids. The afferent and efferent connectivity of the neurons of the SDN-POA will be determined by modern neuroanatomical techniques (autoradiographic analysis of the anterograde transport of tritiated amino acids, and the retrograde transport of the fluorescent marker Evans Blue) as well as by the electrophysiological approach. Finally, the temporal pattern of development of these connections, and their possible influence on the SDN-POA will be determined. The process of sexual differentiation of the brain is critical for normal sex specific patterns of neuroendocrine regulation and the present studies will elucidate the influence of gonadal steroids on fundamental processes (the mitotic division of neuroblasts, the migration, survival and maturation of neurons) which underly the functional sex differentiation of the brain.